1. Field of the Invention
The present invention relates to a thrust fin for ships that recovers lost energy caused by rotational flow, which is generated behind a propeller, and converts it into thrust.
2. Description of Related Art
As well known to those in the art, a propeller of a ship generates thrust using its rotation, but rotational flow is generated behind the propeller while the propeller rotates, as shown in FIG. 1. The rotational flow is irrelevant to the generation of the thrust, but merely causes energy loss. Thrust fins are one type of device that enhances propulsive efficiency of the ship through the recovery of a large amount of energy loss caused due to the rotational flow that is generated behind the propeller.
Such a thrust fin for a ship generates lift in the direction of the movement of the ship to enhance propulsive efficiency of the ship, in the same manner as that of the wings of an airplane. The thrust fin serves to generate strong lift, that is, strong thrust in the slipstream of a propeller, in which rotational flow is generated, while the speed of an axial current is increased by the propeller. Therefore, the thrust fin is preferably mounted to a rudder placed behind the propeller.
However, because this region has complex flow characteristics compared to other regions, it is very difficult to optimize an angle between the fin and a stream of fluid, that is, an angle of attack.
Meanwhile, the propeller, which rotates to move the ship, accelerates the surrounding fluid. An axial acceleration factor of the fluid generates thrust, but an acceleration factor, related to a direction of rotation of the propeller, merely causes energy loss, without helping the thrust to increase.
To recover lost energy caused by the acceleration factor related to the direction of rotation of the propeller, additional devices of various shapes have been developed and used in a rear portion of the body of the ship. However, most conventional additional devices are problematic in that they can improve propulsive efficiency only in a specific ship and under voyage conditions. Furthermore, due to the complex structure, installation costs are high, so that the possibility of application to a real ship is reduced.
However, the above-mentioned thrust fin, which is mounted to the rudder of the ship, can markedly enhance propulsive efficiency of the ship despite having a simple structure. Furthermore, because installation of the thrust fin is simple, it is easily applied to a real ship. Hence, a method for optimizing an angle of attack to increase thrust using a thrust fin was proposed by the inventor of the present invention (in Korean Patent Application No. 1988-0018041).
Because the thrust fin can generate strong thrust in the slipstream of the propeller, in which fluid is accelerated by the propeller, the thrust fin is typically mounted to the rudder of the ship in a shape where two fins are provided on respective left and right sidewalls of the rudder, as shown in FIGS. 2A and 2B. Furthermore, the slipstream of the propeller has an asymmetric flow characteristic due to an influence of the body of the ship. Therefore, the left and right sides of the thrust fin must be designed in an asymmetric shape corresponding to the asymmetric flow characteristics, thus markedly enhancing thrust.